The present invention generally relates to digital telecommunications systems. More particularly, the present invention relates to the transmission of information from a transmitter to a receiver without increasing the bandwidth requirement and without introducing transmission delays.
Digital telecommunications systems typically employ one or more modulation schemes to communicate information such as voice, data and/or control information. These modulation schemes may include, GMSK (Gaussian Minimum Shift Keying), M-ary QAM (Quadrature Amplitude Modulation), or M-ary PSK (Phase Shift Keying), where M denotes the number of modulation symbols specified for the different modulation schemes. The different modulation symbols correspond to different information symbols to be transmitted. In M=2 modulation schemes, for example, there are only two different modulation symbols specified. Hence, an M=2 modulation scheme is referred to as a binary modulation scheme.
The different types of modulation may be affected differently by the quality of the communication channel, i.e., different schemes may be more or less susceptible to distortion, time dispersion C/I ratios and the like. Accordingly, it is said that different modulation schemes have different levels of robustness. Generally, as the number of modulation symbols increases, i.e., as the value of M increases, the modulation scheme tends to be less robust. There are, however, other factors that influence the robustness of a modulation scheme, for example, the symbol rate. The symbol rate may also be specified differently for a given modulation scheme as well as between different modulation schemes.
In order to assure adequate communication quality with respect to e.g., information bit rates and error rates, link adaptation may be utilized. Depending on the (time-varying) quality of the communication channel, which may be affected by, for example, noise level, interference, path loss and time dispersion, a link adaptation strategy assures that an appropriate modulation scheme, channel coding, source coding, bandwidth and signal power level are chosen to obtain a link quality that satisfies user demands in terms of error rates, throughput and the like. To be truly effective, the link adaptation technique must be capable of monitoring and/or measuring the channel conditions over relatively short periods of time. Then, based on the present channel conditions, the system selects the modulation scheme or schemes that optimize link quality.
Telecommunications systems that employ Time Division Multiple Access (TDMA) divide the available frequency band into several RF channels. Each of these RF channels are then further divided into several physical channels or time slots. Voice, data and/or control information is then transmitted in bursts, wherein a burst corresponds to a physical channel or time slot. In a TDMA based system, link adaptation and modulation selection is typically accomplished on a burst-by-burst basis. It will be understood, however, that link adaptation is not limited to TDMA systems. Rather, link adaptation may also be performed in systems based on other access principles. For example, in CDMA (Code Division Multiple Access) Systems, one may vary, e.g., coding, modulation and spreading factors to achieve a desired link quality.
An important aspect of any link adaptation and modulation selection technique is the way in which the transmitter informs the receiver of the modulation scheme selected for a particular burst of information. Probably, the most straight-forward technique for informing the receiver as to the modulation scheme associated with a particular burst of information involves signaling the receiver in advance. However, this technique is highly undesirable as it results in additional overhead (i.e., an increase in the bandwidth requirement) which, in turn, results in transmission delays.
Another technique for conveying modulation selection information to the receiver involves the use of training sequences. As one skilled in the art will readily appreciate, training sequences are typically employed at the receiver for estimating the distortion and noise characteristics of a channel. For example, upon receiving a training sequence, the receiver compares the values associated with the received training sequence to the values associated with an expected training sequence. The receiver then utilizes the difference to characterize the channel (i.e., estimate the channel).
In order to use the training sequences to convey modulation selection information to the receiver, one or more training sequences must be assigned to each of the various modulation schemes. However, this solution also has a number of disadvantages. Foremost is the fact that it is difficult to identify an adequate number of unique training sequences with good auto correlation properties. Also, additional memory is required to store each of the additional training sequences. Furthermore, additional control software is needed to handle the additional training sequences.
Ideally, the receiver should be able to determine the modulation scheme associated with a particular burst of information without advanced signaling from the transmitter, as advanced signaling introduces bandwith loss and transmission delays. Also, the receiver should be able to determine the modulation scheme during the channel estimation process (i.e., prior to the equalization process), as the equalization process is complex and modulation dependent. Furthermore, the receiver should be able to detect the modulation scheme independent of the fact that each, or at least two or more, modulation schemes employ the same training sequence and symbol rate.
The present invention involves a technique which allows a transmitter in a telecommunications system to transmit signaling information, such as information relating to modulation format, to a receiver without increasing the transmission bandwidth and without introducing any significant transmission delays. In general, the present invention accomplishes this by employing a symbol constellation phase rotation technique.
Accordingly, it is an object of the present invention to convey transmission information, such as modulation information, to a receiver in a telecommunications system without advanced signaling from the transmitter.
It is another object of the present invention to convey transmission information to a receiver in a telecommunications system without increasing overhead (i.e., without increasing the bandwidth requirements).
It is still another object of the present invention to convey transmission information to a receiver in a telecommunications system, wherein the receiver recognizes the information prior to equalization and independent of the training sequences and symbol rates used during channel estimation.
In accordance with one aspect of the present invention, the foregoing and other objects are achieved by a method and/or apparatus for transmitting signaling information from a transmitter to a receiver. The method and/or apparatus involves identifying one of a plurality of information signals to be conveyed from the transmitter to the receiver in addition to data and then rotating each of one or more symbols by a common phase rotation factor, wherein the phase rotation factor uniquely identifies the one information signal to be conveyed from the transmitter to the receiver. Each of the one or more phase-rotated symbols is then transmitted to the receiver.
In accordance with another aspect of the present invention, the foregoing and other objects are achieved by a method and/or apparatus for conveying modulation information from a transmitter to a receiver. The method and/or apparatus involves selecting one of a plurality of modulation schemes and modulating a sequence of training symbols in accordance with the selected one of the plurality of modulation schemes. A phase rotation factor is then identified which corresponds to the selected one of the plurality of modulation schemes, wherein at least one unique phase rotation factor is associated with each of the plurality of modulation schemes. The phase of each training symbol is then rotated as a function of the identified phase rotation factor which corresponds to the selected one of the plurality of modulation schemes, and the sequence of phase-rotated training symbols are transmitted to the receiver. At the receiver, a sequence of de-rotated training symbols is generated for each phase rotation factor by de-rotating the received sequence of phase-rotated training symbols as a function of each phase rotation factor. Each sequence of de-rotated training symbols is then compared to an expected sequence of training symbols, and the sequence of derotated training symbols that approximates the expected sequence of training symbols most accurately is identified. Finally, the selected one of the plurality of modulation schemes is identified based on the phase rotation factor that produced the sequence of de-rotated training symbols that most closely approximated the expected sequence of training symbols.
In accordance with yet another aspect of the present invention, the foregoing and other objects are achieved by a method and/or apparatus for identifying signaling information transmitted from a transmitter to a receiver along with data. The method and/or apparatus involves receiving a signal containing a sequence of symbols transmitted from the transmitter, wherein the symbols include training symbols and data symbols, and wherein the phase associated with each symbol in the sequence of symbols has been rotated at the transmitter in accordance with a common phase rotation factor that corresponds to the signaling information. Samples from the received signal are then generated, wherein the samples correspond to the sequence of transmitted symbols. The sequence of transmitted symbols is then recovered by de-rotating the samples in accordance with the common phase rotation factor; and the signaling information is identified as a function of the common phase rotation factor used to recover the sequence of transmitted symbols.